(Health.com) -- For years, dermatologists have been aware of -- and baffled by -- people who feel a constant creepy-crawly sensation beneath their skin, which they believe is due to bugs, worms, or eggs below the surface.

Now, in the largest study to date to examine skin samples from patients with these symptoms, doctors have firm proof that these infestations -- known as delusional parasitosis or delusional infestation -- are not real.

The researchers acknowledge, however, that the findings may not be enough convince many of these patients. Patients often feel dismissed when doctors reassure them that the infestation is all in their head, and many continue to believe they are teeming with bugs even when skin biopsies come back negative.

"It's almost impossible to get them to shake this belief, no matter how much evidence you produce to the contrary," says Mark D. P. Davis, M.D., a professor of dermatology at the Mayo Clinic, in Rochester, Minnesota.

Antipsychotic drugs are the standard treatment for delusional infestation. But, Davis says, "A lot of patients with this disorder don't want to take these drugs because they don't feel they have a delusional disorder."

Some patients who experience this skin-crawling sensation believe it is caused by textile-like fibers produced by an unknown organism.

Along with a group of sympathetic doctors and advocates, these patients have pushed for the condition to be officially recognized as Morgellons disease, and have lobbied -- successfully -- for the Centers for Disease Control and Prevention to investigate it.

However, most doctors maintain that the condition is psychological rather than physical.

In a new study, published this week in the Archives of Dermatology, Davis and his colleagues sought to confirm this view by presenting the results of skin biopsies taken from patients in whom the delusional infestation was diagnosed at the Mayo Clinic between 2001 and 2007.

The researchers performed 80 biopsies. As expected, none showed evidence of skin infestation, although 49 patients did have some skin inflammation, known as dermatitis.

This inflammation might be due to some underlying cause, such as allergies, or it could have been caused by the patient's efforts to remove the bugs or objects by digging them out or even trying to burn them, Davis says.

In addition, 80 of the study participants -- including some who also had biopsies taken -- supplied their own skin samples to the doctors.

Ten of these specimens contained insects, such as a mite or tick, but only one such bug was actually capable of causing an infestation; it was a pubic louse, but the patient's biopsy did not show any signs that his or her skin was infested with the lice.

The CDC recently completed its own study of the condition, which the agency refers to as unexplained dermopathy, but the results have not yet been published.

Davis' study "leaves little room for speculation," says Roland Freudenmann, M.D., an associate professor of psychiatry at the University of Ulm, in Germany. Freudenmann is an authority on delusional infestation but didn't take part in the current study.

"It may irritate those who cannot accept the diagnosis of delusional infestation," he says. "But the study shows that delusional infestation exists and that it is the patients who are wrong."

Freudenmann adds that he hopes the new findings, as well as the forthcoming CDC report, "bring an end to the 'Morgellons story.'" Jennifer Murase, M.D., a dermatologist and associate clinical professor at the University of California San Francisco (UCSF), says she typically sees a handful of patients with delusional infestation each year.

It's important to take the patient's complaints seriously, says Murase, who was not involved in the new research. These people often have an underlying skin condition such as eczema or psoriasis, which could be affecting their nerves in abnormal ways and contributing to the sensation of infestation, she explains.

Murase and her colleagues often conduct biopsies in these patients, mostly to reassure the patients rather than to confirm the diagnosis of delusional infestation.

The patient chooses the spot where the biopsy is to be taken, and must agree that only one biopsy will be done. The patient also has to agree that if the microscopic examination doesn't find evidence of infestation, "they will be more open-minded and think of other causes for the condition," she says.

John Koo, M.D., a dermatologist who works with Murase at UCSF, estimates that roughly one-third of these patients can be easily convinced that the problem is not a real infestation; one-third require more convincing; and another one-third can't be swayed from their conviction.

"If you kind of hang in there and stay with them, they can get better," Murase says. "They can actually do quite well."